In the known art, cigarettes usually enter a packeting machine by way of a hopper, to which the cigarettes are fed by suitable containers or by means of a belt conveyor in the form of a continuous layer. At the outlet of said hopper the cigarettes are combined into groups each constituted by a number of cigarettes equal to the number in a finished packet.
The characteristics of the cigarettes of each group are then checked, and those groups which comprise even only one defective cigarette are discarded.
In order to reduce the number of expelled groups of cigarettes, and thus attain considerable economical saving, a device has been proposed by the present applicant G. D. S.p.A. in U.S. Pat. No. 4,376,484, which is able to check the regularity of the cigarettes while they are still in the hopper, and to discard them before they reach the grouping station. In this device, the cigarettes are expelled from the hopper preferably by means of a pneumatic expulsion device, which directs an axial stream of compressed air against the cigarettes found to be defective, causing them to emerge through suitable apertures provided in the hopper itself.
A device of this type is however not free from drawbacks, it having been found that during their exit from the hopper the defective cigarettes frequently cause disarranging of the cigarettes lying above and below them. In this respect, the cigarettes lying above the defective cigarettes often give rise to flooding to the extent of requiring the stoppage of the packeting machine in that following the sudden exit of the immediately underlying cigarettes, they undergo a sudden uncontrolled descent during which they frequently assume an irregular arrangement.
The cigarettes lying below the discarded cigarettes also tend to become irregularly arranged during expulsion of the defective cigarettes, because of the generation of air vortices which suck them upwards in a disordered manner.
Because of these vortices, it also frequently happens that a cigarette adjacent to a defective cigarette is sucked out of the hopper, through one of the said apertures, during the described expulsion stage.
A further drawback of the described device derives from the fact that the level of the cigarettes which during each operating cycle reach the checking position and expulsion position is not rigorously constant. These level variations are caused by uncontrollable variations in the diameter of the cigarettes and also by tobacco dust or fragments which become interposed between them.
If the aforesaid events should occur, the checking device and the expulsion device exercise their action not properly in the central zones of the cigarette ends, but rather in peripheral zones thereof or even in the zones of contact between two cigarettes.